Chemical Mechanical Polishing (CMP) slurry, together with polishing pads and diamond conditioner disks form the key components of the equipment used to carry out CMP processes in recent years. These polishing pads and diamond conditioner disks have been produced and marketed by several vendors to standards of reliable quality and effectiveness. The function of the polishing pad is to cut away and polish the wafer surface in conjunction with the slurry. As they accomplish this function, the polishing pads themselves become smooth and lose effectiveness in their capacity to polish the wafer surface. The function of the diamond conditioner discs, the surface facing the polishing pad of which is covered with small embedded diamonds or other hard substance, is to cut into and roughen the polishing pad surface during polishing so that it is continually being roughened as the wafer smooths it. This way the effectiveness of the polishing pad is maintained constant. The function of the slurry is to deliver continuously the mechanical abrasive particles and chemical components to the surface of the wafer and to provide a means of removing reaction products and wafer debris from the polishing surface. There are several varieties of slurry of varying effectiveness and properties known to the art. At present, for the most common type of CMP tool, the rotary polisher, slurry is applied at a constant flow rate onto the rotating polishing pad using a simple delivery tube, nozzle or spray bar. Fresh slurry flows away from the application point(s) under the influence of gravity and centripetal acceleration and becomes mixed with used slurry or slurry that has passed between the polishing pad and wafer and been involved in polishing.
Old slurry besides being chemically “spent” additionally contains the debris from wafer, conditioner and pad which if the old slurry reenters the gap between the wafer and polishing pad are exposed to the wafer surface and can lead to increases in contamination and defectivity. It is therefore important to remove the debris of polishing, and by extension used slurry, from the polishing pad quickly after it is generated and to the greatest extent possible not reintroduce it under the wafer.
Eventually the rotation of the pad brings the slurry into contact with the leading edge of the wafer, where it forms a bow wave. Some of the fresh slurry at this point is advected into the narrow 10 to 25 micron gap between the wafer and polishing pad and is utilized for polishing. The gap exists because the surface of the pad is rough, the surface of the wafer is relatively smooth and the wafer contacts only the high points of the pad surface. However, most of the fresh slurry remains in the bow wave and is carried to the edge of the pad by the combined rotation of the polishing head and pad. The slurry is then lost over the edge of the pad. Thus, actual slurry utilization, the percentage of new slurry applied that enters the gap between the rough pad surface and the wafer of total slurry applied, is universally quite low in such rotary CMP tools. This is a significant problem because slurry consumption and waste disposal account for a large share of the cost of ownership and operation of a CMP tool.
An additional negative influence on polishing removal rate and uniformity arise because when wafers are polished it is the practice in the art to wash used slurry off between wafers by application of deionized water to the pad, typically to the center of the pad. The time between removing one wafer and replacing it with a second is short and invariably a large quantity of water remains on the pad when polishing of the new wafer begins. This water is not uniformly distributed and as a result it dilutes the newly added slurry in a non-uniform way causing both general decrease in removal rate by the diluted slurry and lack of uniformity in removal rate due to variations in slurry concentration on different parts of the pad. Since this effect lasts several seconds it can exert a significant negative effect on anywhere from 25 percent to 50 percent of the time during which the wafer is polished resulting in a significant and costly reduction in process effectiveness and product quality.
To facilitate the advection or entry of the slurry under the wafer, the practitioners of the prior art have used grooves in the CMP pad. This was effective in making sure that some slurry reached the pad-wafer interface but still allowed most of the slurry to be cast off of the pad without ever having been used. Slurry is expensive and devices, equipment and procedures for providing and removing large amounts of slurry must be included in the CMP process which both complicates and encumbers that process. Presently there is no effective method available for substantially reducing the amount of slurry used or making sure that most of the slurry introduced to the pad during CMP is actually introduced between the pad and the wafer and utilized as intended before being cast off of the pad.
Methods to solve this problem to date have, as stated above, consisted of placing grooves in the surface of the CMP pad to conduct some portion of the slurry under the wafer during CMP polishing. In U.S. Pat. No. 5,216,843 (Breivogel et al filing date 24 Sep. 1992 hereby incorporated by reference) “an apparatus for polishing a thin film” . . . “said apparatus comprising” . . . “a pad covering said table, said pad having an upper surface into which have been formed a plurality of preformed grooves, said preformed grooves facilitating the polishing process by creating a corresponding plurality of point contacts at the pad/substrate interface.” and a “means for providing a plurality of micro channel grooves into said upper surface of said pad while polishing said substrate wherein said microchannel grooves aid in facilitating said polishing process by channelling said slurry between said substrate and said pad.” Still in U.S. Pat. No. 7,175,510 (Skyopec et al. filing date 19 Apr. 2005 hereby incorporated by reference) a method of polishing wherein “The polishing pad has grooves that channels (sic) slurry between the wafer and polishing pad and rids excess material from the wafer, allowing an efficient polishing of the surface of the wafer.” is described. Even as recently as Skyopec et al the preferred method for maximizing the amount of slurry that was introduced between the pad and the wafer was preparation of the grooves and the efforts of practitioners of the art were limited to ensuring that these “micro-channels” were regenerated or maintained in a suitable fashion.
In US 2007 0224920 (hereby incorporated by reference) these grooves are enhanced by holes in the pad made in sizes and shapes appropriate to optimize the amount of slurry conducted under the wafer by the grooves. However this does not solve the basic problem of waste of new slurry due to slurry accumulation in the bow wave.
Moreover, Novellus Systems, Inc. has addressed the slurry utilization problem by means of orbital polishers (U.S. Pat. No. 6,500,055 hereby incorporated by reference) in which the slurry is injected through the polishing pad directly under the wafer (U.S. Pat. No. 5,554,064 hereby incorporated by reference). This guarantees high slurry utilization but requires a complex platen and custom pad to accommodate the slurry distribution system and a specialized polishing tool to take advantage of the injection method. Similarly in US 20070281592 (hereby incorporated by reference) slurries and other conditioning chemicals are introduced and removed through apertures in the diamond conditioning disk for the purpose of facilitating multistep CMP processes but this is not intended to and does not effectively improve the utilization of slurry by directing a larger fraction between the wafer and the CMP pad.
Also in the prior art are U.S. Pat. No. 5,964,413 (hereby incorporated by reference), which teaches an Apparatus for dispensing slurry. This is a device for spraying slurry on to the pad rather than pumping it in specific positions at the pad wafer interface and does not provide the desirable benefits sought by the present invention.
In addition, U.S. Pat. No. 6,929,533, (hereby incorporated by reference) teaches methods for enhancing within-wafer CMP uniformity. This patent describes methods for enhancing the polish rate uniformity of rotary and linear polishers using slurry dispense bars with multiple nozzles to distribute the slurry over the entire wafer track. The slurry dispense bars sit above the pad and do not contact it. This method when compared with the present invention lacks the effect of the creation of a layer of slurry with the same thickness as the wafer-pad gap which allows significant amounts of the new slurry to be advected under the pad the first time.
U.S. Pat. No. 6,283,840 (hereby incorporated by reference) teaches a cleaning and slurry distribution system assembly for use in chemical mechanical polishing apparatus. This apparatus has “an outlet to distribute slurry to the enclosed region to form a reservoir of slurry in the enclosed region, wherein the slurry is distributed to a region not enclosed by the retainer by traveling between the polishing surface and the lower surface of the retainer.” However, the application of the slurry to specific land areas where it is needed is not taught and in fact most slurry is lost through grooves between the land areas which generally exceed the land areas in cross sectional area between the wafer and the polishing pad. This apparatus also fails to teach or accomplish control over flow as a function of radius from the center of the polishing pad and there is no teaching or reported effect of separation of the old spent slurry, dilution water or polishing wastes from the newly applied slurry. The main function that the apparatus accomplishes is to keep spray from the slurry or from cleaning agents from depositing on the polisher, where the residue can become a source of defect-causing contamination. This is mentioned several times in the description. The background mentions reducing slurry consumption in passing in the last paragraph, but the patent contains no teaching that the apparatus accomplishes this or indeed how it would be accomplished.
U.S. Pat. No. 5,997,392 (hereby incorporated by reference), teaches Slurry injection technique for chemical-mechanical polishing. The slurry application method involves spraying the slurry onto the pad under pressure from a multiplicity of nozzles, however, this invention suffers from the same drawbacks as U.S. Pat. No. 6,929,533 (hereby incorporated by reference) in that lack of precision in the placement and form of the slurry substantially decreases its effectiveness.
“U.S. Pat. No. 4,910,155 (hereby incorporated by reference) describes the basic CMP process and utilizes a retaining wall around the polishing pad and polishing table to retain a pool of slurry on the pad. It does not describe a particular method for getting the pooled slurry into the pad wafer gap more effectively. U.S. Pat. No. 5,403,228 (hereby incorporated by reference) discloses a technique for mounting multiple polishing pads onto a platen in a CMP process. A seal of material impervious to the chemical action of the polishing slurry is disposed about the perimeter of the interface between the pads and when the pads are assembled the bead squashes and forms a seal and causes the periphery of the upper pad to curve upward creating a bowl-like reservoir for increasing the residence time of slurry on the face of the pad prior to overflowing the pad.
U.S. Pat. No. 3,342,652 (hereby incorporated by reference) teaches a process for chemically polishing a semiconductor substrate and a slurry solution is applied to the surface of the pad in bursts as a stream forming a liquid layer between the cloth and the wafers to be polished. The solution is applied from a dispensing bottle and is applied tangentially to the wafer-plate assembly so as to provide maximum washing of the polishing cloth in order to remove waste etching products. U.S. Pat. No. 4,549,374 (hereby incorporated by reference) shows the use of a specially formulated abrasive slurry for polishing semiconductor wafers comprising montmorillonite clay in deionized water.”
U.S. Pat. No. 6,284,092 (hereby incorporated by reference), teaches a CMP slurry atomization slurry dispense system in which “ . . . a polishing slurry dispenser device disposed to dispense the slurry toward the pad preferably as a stream or more preferably drops toward the pad surface and a curtain of air to intersect the slurry at or near the polishing pad surface. The wafer is polished using less slurry than a conventional polishing apparatus while still maintaining the polishing rates and polishing uniformity of the prior art polishing apparatus. A preferred dispenser is an elongated housing having a slurry tube and air tube therein each tube having a plurality of spaced apart slurry openings and air openings along its longitudinal axis which tube is preferably positioned radially over at least one-half the diameter of the polishing pad. A polishing slurry is directed from the slurry tube toward the surface of the pad, preferably in the form of drops, and the air from the air tube forms an air curtain, with the air curtain intersecting the slurry drops preferably at or slightly above the pad surface to atomize the slurry.”
While this system distributes the slurry uniformly it does not do so in a way that insures that the thickness of the slurry layer at the leading edge of the wafer is at or close to the thickness of the gap.
U.S. Pat. No. 6,398,627 (hereby incorporated by reference) teaches a slurry dispenser having multiple adjustable nozzles. In the teaching of that art, a “slurry dispensing unit for a chemical mechanical polishing apparatus equipped with multiple slurry dispensing nozzles is disclosed. The slurry dispensing unit is constructed by a dispenser body that has a delivery conduit, a return conduit and a U-shape conduit connected in fluid communication therein between for flowing continuously a slurry solution therethrough and a plurality of nozzles integrally connected to and in fluid communication with a fluid passageway in the delivery conduit for dispensing a slurry solution. The multiple slurry dispensing nozzles may either have a fixed opening or adjustable openings by utilizing a flow control valve at each nozzle opening. This patent, as with the previous art referred to, possesses no feature that ensures that the thickness of the slurry layer at the leading edge of the wafer is the same as the wafer pad gap.
U.S. Pat. No. 6,429,131 (hereby incorporated by reference) concerns CMP uniformity and teaches improved CMP uniformity achieved by providing improved control of the slurry distribution. Improved slurry distribution is accomplished by, for example, the use of a slurry dispenser that dispenses slurry from a plurality of dispensing points. Providing a squeeze bar between the slurry dispenser and wafer to redistribute the slurry also improves the slurry distribution. This invention can distribute slurry evenly over the pad but does not provide a uniform layer of slurry the thickness of the gap.
However, although the creation and maintenance of grooves and micro-channels are essential for the operation of CMP polishing generally, they still do not afford an efficient means of introduction of slurry between the pad and the wafer whereby most or even a substantial portion of the slurry introduced onto the pad is actually introduced between the pad and the wafer. Furthermore, although a great many methods have been designed for spreading the slurry evenly on the pad none to date have taught a method for preparing a layer of slurry suitably thick for smooth entry into the pad wafer gap. Most of the slurry continues to accumulate in a bow wave of slurry at the leading edge of the wafer which for the most part moves outward along the leading edge to be dumped off of the edge of the pad and wasted. Moreover, used slurry that has been under the wafer and is contaminated returns as the pad is rotated and mixed with the new slurry at the bow wave decreasing significantly the quality of the slurry used in actual CMP and increasing significantly the waste. And finally none of the methods of the prior art have reduced the negative effects on material removal and uniformity of residual slurry cleaning water added between wafers.